1. Field
The present invention relates to a material for an organic photoelectric device and an organic photoelectric device including the same. More particularly, the present invention relates to a material for an organic photoelectric device having thermal stability due to a glass transition temperature (Tg) of 120° C. or more and a thermal decomposition temperature of 400° C. or more, having bipolar characteristics due to good hole and electron transporting properties, and being capable of realizing high efficiency of an organic photoelectric device, and an organic photoelectric device including the same.
2. Description of the Related Art
A photoelectric device is, in a broad sense, a device for transforming photo energy to electrical energy, and conversely, for transforming electrical energy to photo energy. The photoelectric device may be exemplified by an organic light emitting diode, a solar cell, a transistor, and so on.
Particularly, among these photoelectric devices, the organic light emitting device employing organic light emitting diodes (OLED) has recently drawn attention due to the increase in demand for flat panel displays.
The organic light emitting device transforms electrical energy into light by applying current to an organic light emitting material. It has a structure in which a functional organic material layer is interposed between an anode and a cathode.
The organic light emitting diode has similar electrical characteristics to those of light emitting diodes (LED) in which holes are injected from an anode and electrons are injected from a cathode, then the holes and electrons move to opposite electrodes and are recombined to form excitons having high energy. The formed excitons generate lights having a certain wavelength while shifting to a ground state.
Generally, the organic light emitting diode is composed of an anode of a transparent electrode, an organic thin layer of a light emitting region, and a metal electrode (cathode) formed on a glass substrate, in that order. The organic thin layer may includes an emission layer, a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), or an electron injection layer (EIL). It may further include an electron blocking layer or a hole blocking layer due to the emission characteristics of the emission layer.
When the organic light emitting diode is applied with an electric field, the holes and electrons are injected from the anode and the cathode, respectively. The injected holes and electrons are recombined on the emission layer though the hole transport layer (HTL) and the electron transport layer (ETL) to provide light emitting excitons.
The provided light emitting excitons emit light by transiting to the ground state.
The light emitting may be classified as a fluorescent material including singlet excitons and a phosphorescent material including triplet excitons.
In other words, the duration of fluorescent emission is extremely short at several nanoseconds, but the duration of phosphorescent emission is relatively long such as at several microseconds, so that it provides a characteristic of extending the lifetime (emission duration) to more than that of the fluorescent emission.
In addition, evaluating quantum mechanically, when holes injected from the anode are recombined with electrons injected from the cathode to provide light emitting excitons, the singlet and the triplet are produced in a ratio of 1:3, in which the triplet light emitting excitons are produced at three times the amount of the singlet light emitting excitons in the organic light emitting diode.
Accordingly, the percentage of the singlet exited state is 25% (the triplet is 75%) in the case of a fluorescent material, so it has limits in luminous efficiency. On the other hand, in the case of a phosphorescent material, it can utilize 75% of the triplet exited state and 25% of the singlet exited state, so theoretically the internal quantum efficiency can reach up to 100%. When phosphorescent light emitting material is used, it has advantages in an increase in luminous efficiency of around four times than that of the fluorescent light emitting material.
In this structure, the efficiency and properties of the light emission diodes are dependent on the host material in the emission layer. According to studies regarding the emission layer (host), the organic host material can be exemplified by a material including naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, pycene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobi phenyl, trans-stilbene, and 1,4-diphenylbutadiene.
Generally, the host material includes 4,4-N,N-dicarbazolebiphenyl (CBP) having a glass transition temperature of 110° C. or less and a thermal decomposition temperature of 400° C. or less, in which the thermal stability is low and the symmetry is excessively high. Thereby, it tends to crystallize and cause problems such as a short and a pixel defection according to results of thermal resistance tests of the devices.
In addition, most host materials including CBP are materials in which the hole transporting property is greater than the electron transporting property. In other words, as the injected hole transportation is faster than the injected electron transportation, the excitons are ineffectively formed in the emission layer. Therefore, the resultant device has deteriorated luminous efficiency.
Accordingly, in order to realize a highly efficient and long lifetime organic light emitting device, it is required to develop a phosphorescent host material having high electrical and thermal stability and that is capable of transporting both holes and electrons.